This invention relates to a flight control apparatus for the control of the trajectory and of the aerodynamic condition of flow (for example, of the air speed) of airplanes, comprising a measuring arrangement for the aircraft position, a measuring arrangement for the aerodynamic condition of flow, a thrust-changing actuator and at least one actuator for a control surface of the aircraft (for example, the elevator) including inputs for command values for trajectory and aerodynamic condition of flow, further comprising means for forming deviation signals from the signals of the measuring arrangements and of the associated command values, and comprising means for controlling the actuators in dependence on such deviation signals in a sense of reducing the deviations.
In conventional flight control apparatus separate control systems are provided for the aerodynamic condition of flow, such as the air speed relative to the ambient air, and for the trajectory. The air speed is usually derived by means of a dynamic pressures sensor, and is compared to a command value. The deviation signal is applied to an actuator for the throttle and thus effects a change of the engine thrust in a sense counteracting the air speed deviation. The operation of the control system for the attitude control and trajectory control (automatic pilot and damper) is completely independent thereof.
It includes a measuring arrangement for the position of the aircraft, for example, an altimeter. The signal thus obtained is compared to a command value and the deviation signal thus formed is applied to an automatic pilot acting, for example, on the elevator through a servomotor. In case of an elevational deviation the elevator is then actuated so that the aircraft climbs or descends as necessary to correct the deviation.
In addition to automatic pilot, damper and speed controller in some applications, recently a direct lift control (DLC) has been provided for improving the trajectory control, for increasing the touch-down accuracy, for increasing passenger comfort and for reducing aircraft cell load, in which, for example, vertical acceleration and angle of attack signals are applied to a lift-changing regulating means (such as lift flaps or spoilers).
Moreover, it is prior art to apply additional flight state variables to the speed controller and the automatic pilot so as to improve the control performance. Thus, by way of example, in a prior speed controller a longitudinal acceleration signal from a longitudinal accelerometer is connected in opposition to the deviation signal in order to reduce the throttle activity. If the aircraft is subjected to a gust from ahead so that the relative speed between aircraft and ambient air is temporarily increased, the aircraft is simultaneously decelerated with respect to ground whereby a declaration signal is generated. These signals can be made to cancel each other and the control apparatus does not react with throttle activity to such gusts -- similar to the human pilot (British patent specification No. 1,190,199).
Also in other speed controllers longitudinal acceleration signals are applied in order to obtain a damping of the control loop, the longitudinal acceleration signal replacing the otherwise hard-to-form time derivative of the speed signal. A signal proportional to the pitch angle speed and derived from a rate gyro is, for example, applied for damping to that channel of an automatic pilot controlling the servomotor for the elevator.
These prior art flight control apparatus, namely speed control apparatus, direct lift control means (DLC) and automatic pilots suffer from the shortcoming that the control systems separated instrumentally, e.g. for air speed or the like and for trajectory, are intercoupled via the performance of the aircraft. An intervention in one control system causes a disturbance in the other one, and vice versa. By way of example, it shall be assumed that the altimeter indicates too great an elevation with respect to the commanded elevation. The automatic pilot then initiates a descent in order to again obtain the commanded elevation. In so doing, however, potential energy of the aircraft is converted to kinetic energy and the air speed increases. Thus, by the correction of the deviation in the trajectory control system by the automatic pilot a disturbance of the air speed control system is caused. This disturbance must be counteracted by an intervention of the speed controller. Conversely a change in thrust generally involves a change in the trajectory angle so that the aircraft climbs or descends, thus changes its trajectory. It is obvious that such a type of control tends to result in rather large deviations and small damping. It is extremely difficult with such a flight control apparatus comprising a separate speed control apparatus and automatic pilot to guide the aircraft with respect to trajectory and speed with the accuracy required, for example for an automatic STOL-landing.
In order to avoid these difficulties it is known to apply to the speed controller, in addition to the air-speed-deviation signal .alpha. u, a signal proportional to the vertical acceleration or the acceleration in the direction of the aircraft yaw axis. This prior arrangement (British patent specification No. 1,190,198) is intended for a type of aircraft in which a change in thrust primarily affects the trajectory and an elevator adjustment primarily affects the air speed. By the signal proportional to the vertical acceleration h, a deviation from the constant flight altitude or a straight glide path is determined and used for trajectory control on the shortest signal path, namely via the speed controller. Also in this prior arrangement there are a separate speed controller and an automatic pilot, the former only acting on the throttle and the latter acting only on the elevator. Also this prior art apparatus encounters the previously described difficulties which by the additional application of the vertical acceleration to the speed control apparatus are insufficiently alleviated and only for the special case of straight trajectories flown at constant speed.
Modern control theory teaches that an optimum control is obtained if all state variables relevant to the performance of the controlled system are applied in a suitable linear combination to all actuators provided. In practice, however, this theoretical requirement cannot be met in general. In the case of complex control systems the number of the condition variables to be considered becomes too great and part of these condition variables is not readily measurable.
It is an object of this invention to provide a flight control apparatus capable of maintaining a given trajectory -- which might be curved -- and given air speeds of the aircraft with very good accuracy, while providing low throttle activity and high passenger comfort.
It is a more specific object of this invention to reduce the influence of the air speed control on the trajectory control, and vice versa, which is particularly great in STOL-airplanes having great lift factors.
In accordance with the broad concept of the invention there is apparatus for controlling the trajectory and aerodynamic condition of flow of an airplane having thrust changing actuator means and control surface actuator means. Said apparatus comprises trajectory sensor means for producing an acutal trajectory signal, aerodynamic condition of flow sensor means for producing an actual aerodynamic of flow signal, trajectory deviation detector means having a command value input and being connected to the trajectory sensor means to produce a trajectory deviation signal indicative of the deviation of the actual trajectory signal from a commanded value, aerodynamic condition of flow deviation detector means having a command value input and being connected to the aerodynamic condition of flow sensor means to produce an aerodynamic condition of flow deviation signal indicative of the deviation of the actual aerodynamic condition of flow signal from a command value, and control means connected to the actuator means and to the deviation detector means for controlling the actuators in accordance with said deviation signals. Said control means control the thrust changing actuator means in response to first control signals, which are related to the signals from both of the deviation detector means by given associated transfer functions, and control the control surface actuator means in response to second signals which are related to the signals from both of the deviation detector means by given associated transfer functions.
The transfer functions may be just coefficients, the signals from both of the deviation detector means being applied directly with these coefficients to both of said actuator means. Alternatively, the signals from both of said deviation detector means are applied to both of the actuator means through filters, i.e. with appropriate transfer functions, as the characteristics of the system may require.
Said coefficients or transfer functions may be selected to reduce the mutual influence of the two controlled variables, namely trajectory and aerodynamic condition of flow.
When applying the deviation signal for the trajectory (for example, the elevational deviation) only to the actuator for the associated control surface (for example, the elevator), then, as described above, in addition to the trajectory also the aerodynamic condition of flow (for example, the air speed or the angle of attack) is influenced thereby. On the other hand, when applying the deviation signal only to the thrust-determining actuator (throttle) the trajectory as well as the air speed can be influenced thereby. Therefore, the signal from the measuring arrangement for the trajectory, both when applied to the thrust-determining actuator and also when applied to the actuator for the elevator, for example, also influences the aerodynamic condition of flow and the air speed. By simultaneously applying the trajectory deviation signal with appropriate factors and appropriate sign to the thrust-determining actuator and to the respective control surface (elevator) actuator, it is possible to substantially compensate the two influences of the trajectory deviation signal on the aerodynamic condition of flow. Thus, the combined intervention has no effect on the aerodynamic condition of flow. However, there is a resultant influence on the trajectory counteracting the deviation.
The same considerations apply to a deviation of the aerodynamic condition of flow. An application of the aerodynamic condition of flow deviation signal to the throttle only does not only influence the aerodynamic condition of flow but also the flight altitude via a change in the angle of attack and, consequently, in the lift. An application of the deviation signal to the elevator alone does not only cause a change in the air speed -- via the conversion of potential energy to kinetic energy or vice versa -- but, of course, also a change in the flight altitude. The aerodynamic condition of flow deviation signal can be applied both to the throttle and also to the elevator with such factors and sign that its effects on the flight altitude are completely or substantially compensated. However, a resultant influence on the air speed is maintained. By using such an instrumentally integrated flight control apparatus for trajectory and aerodynamic condition of flow, the control operations for these quantities can actually be decoupled, leading to a substantial improvement of the control accuracy, of gust and thermal wind suppression, to reduced throttle activity and improved passenger comfort. At the same time the integrated flight control apparatus permits a greater freedom in the selection of the control parameters for the control of each individual controlled variable.
It is already advantageous if the factors or transfer functions are selected to reduce the mutual influence of the control operations. Preferably, however, the coefficients of the linear combinations and the filter time constants are selected so that a deviation of the trajectory and its correction is substantially without any influence on the aerodynamic condition of flow and a deviation of the aerodynamic condition of flow and its correction is substantially without any influence on the trajectory.
In accordance with another aspect of the invention, a flight control apparatus for automatically controlling the position and the aerodynamic condition of flow of an aircraft comprises devices for producing state variable signals including first measuring means for producing a first signal indicative of aircraft position, such as altitude, second measuring means for producing a second signal indicative of aerodynamic condition of flow, such as angle of attack, first command signal generating means for producing an aircraft position command signal, second command signal generating means for producing an aerodynamic condition of flow command signal, a first comparator means for comparing said first signal and said aircraft position command signal to produce a position deviation signal, second comparator means for comparing said second signal and said aerodynamic condition of flow command signal to produce a flow condition deviation signal. A thrust actuator has an input and is adapted to vary the thrust of the aircraft in response to signals at the input thereof. A control surface actuator has an input and is adapted to vary the deflection of a control surface of the aircraft, such as the elevator, in response to signals at the input thereof. Circuit means are connected to said devices and the inputs of said actuators for applying a plurality of signals at each of said inputs as linear combinations, each signal of said plurality of signals being indicative of one state variable. Said plurality of signals includes said position deviation signal, said flow condition deviation signal, the time integral of said position deviation signal, the time integral of said flow condition deviation signal and a signal indicative of vertical speed. Said circuit means comprise at least one of each of the following items:
a. direct connection means for directly applying a state variable signal as one signal out of said plurality and
b. filter means connected to receive one of said state variable signals and from it to produce another state variable signal which then is applied to said inputs as being one of said plurality of signals.
It has been found that a selection of state variables sufficient to provide the accurate control of trajectory and aerodynamic condition of flow of an aircraft can be derived either by direct measurement or by appropriate filtering of directly measured quantities. The state variables thus obtained are applied in linear combinations to both the thrust and aerodynamic condition of flow actuators.
Preferably each of the state variables is also applied to a servomotor actuating a control surface, such as a spoiler, for direct lift control.